The present invention relates to thrust bearings used in transmissions of, for example, automobiles, general industrial machines, transport equipment, etc., and toroidal type continuously variable transmissions using the same.
As shown in FIG. 3, a toroidal type continuously variable transmission 50 comprises an input disk 52 and an output disk 53 opposed to each other, a power roller 60 interposed between the disks 52 and 53, a push mechanism (not shown) for pressing the disks 52 and 53 toward each other, etc. The input disk 52 is rotated around an axis P1 by means of a power source such as an engine. The rotation of the input disk 52 is transmitted to the output disk 53 through the power roller 60.
A trunnion 58 is provided between the input and output disks 52 and 53. As shown in FIG. 5, the trunnion 58 has a U-shaped cross section and includes pivots 57a and 57b at its opposite ends and an outer race supporting portion 59 between the pivots 57a and 57b. The trunnion 58 is located between the disks 52 and 53 for rocking motion around the pivots 57a and 57b. 
The power roller 60 shown in FIG. 3 is rockably supported between the input and output disks 52 and 53 by the trunnion 58. The roller 60 is in rolling contact with both the disks 52 and 53. The power roller 60 is rotatably supported in the trunnion 58 by means of a power roller bearing 61 that serves as a thrust bearing.
As shown in FIG. 5, the power roller bearing 61 comprises an inner race including an end face 60a of the power roller 60, an outer race 63 supported in the trunnion 58, and balls 62 arranged between the roller 60 and the race 63 for rolling motion. Annular raceway grooves 64 and 65 in contact with balls 62 are formed, respectively, on end faces 60a and 63a of the power roller 60 and the outer race 63 that face each other. The respective raceway grooves 64 and 65 of the power roller 60 and the outer race 63 are formed having equal shapes (arcuate cross sections) such that the respective centers of the balls 62 are situated in the middle between the end faces 60a and 63a. 
The power roller 60 is designed so that its tilt angle can be changed according to the reduction ratio of the toroidal type continuously variable transmission 50. The power roller 60, which serves as the inner race of the power roller bearing 61, is considerably thicker than the inner race of a conventional thrust bearing.
The toroidal type continuously variable transmission 50 is provided with the push mechanism for pressing the input and output disks 52 and 53 toward each other. The input disk 52 is rotated by means of the power source, and its rotation is transmitted to the output disk 53 through the power roller 60. As the push mechanism presses at least one of the disks 52 and 53, whereupon the disks 52 and 53 come into rolling contact with the power roller 60. As this is done, the power roller bearing 61 allows the power roller 60 to rotate while supporting a load in the thrust direction that acts on the roller 60.
In this toroidal type continuously variable transmission 50, a relatively heavy thrust load acts on the power roller 60 and the power roller bearing 61 during power transmission. This thrust load causes the outer race supporting portion 59 of the trunnion 58 to undergo elastic deformation such that it separates from the power roller 60, as indicated exaggeratedly by broken line Q in FIG. 5. If the trunnion 58 is deformed in this manner, the outer race 63 that is supported by the trunnion 58 is also deformed, so that the respective raceway grooves 64 and 65 of the power roller 60 and the outer race 63 cease to be able to face each other entire. Thereupon, the raceway track of the balls 62 tends to be deviated from the raceway grooves 64 and 65.
Accordingly, thrust loads that act on the raceway groove 65 of the outer race 63 through the balls 62(A) to 62(H) are uneven with respect to the circumferential direction of the outer race 63, as indicated by segments F1, F2 and F3 in FIG. 4. FIG. 4 shows three magnitudes of thrust loads F1, F2 and F3 that act on the outer race 63 when the thrust load on the power roller 60 is varied in three stages.
As seen from FIG. 4, the maximum thrust load acts on the outer race 63 through the balls 62(A) and 62(E) that are situated near the pivots 57a and 57b of the trunnion 58. The thrust loads F1, F2 and F3 lower as the balls 62 approach the axis P1, starting from the balls 62(A) and 62(E), and the minimum thrust load acts on the outer race 63 through the balls 62(C) and 62(G) that are situated near the axis P1. If the thrust load is thus uneven in the circumferential direction of the outer race 63, a part of the raceway surface of the outer race 63 may suffer flaking or the like, possibly lowering the life performance of the race 63 and the toroidal type continuously variable transmission 50 itself.
The raceway surface of the outer race 63 can be prevented from flaking by improving the stiffness of the race 63. Since the outer race 63 of the toroidal type continuously variable transmission 50 is supported by means of the trunnion 58, however, it is structurally hard to enhance the stiffness by increasing the thickness of the race 63. Conventionally, there is a proposal that the stiffness of the outer race 63 should be improved by enhancing the stiffness of the trunnion 58 in order to prevent the flaking on the raceway surface of the outer race 63. If the stiffness of the trunnion 58 is enhanced, however, the trunnion 58 is inevitably large-sized, so that the transmission 50 itself is also large-sized.